drm/vmwgfx: Emulate legacy shaders on guest-backed devices v2

Command stream legacy shader creation and destruction is replaced by
NOPs in the command stream, and instead guest-backed shaders are created
and destroyed as part of the command validation process.

v2: Removed some stray debug messages.

Signed-off-by: Thomas Hellstrom <thellstrom@vmware.com>
Reviewed-by: Jakob Bornecrantz <jakob@vmware.com>